Mass

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Discovery Methods

SDSS J0927+2943

Supermassive black holes are born in the cores of galaxies, but they don't have to stay there. When two black holes merge they may receive a gravitational "kick" strong enough to expel them from the galaxy and send them careening through space alone.

That appears to be the case with SDSS J0927+2943, a quasar (a large, bright disk of hot gas around a supermassive black hole) first seen in a sky survey in the early 2000s. More recent observations suggest it is tens of thousands of light-years from the core of a nearby galaxy, and moving away from the galaxy at more than 5 million miles per hour (2,350 km/sec).

In photographs, the quasar/galaxy combination looks like a single bright object. Spectra, however, show that it is two separate objects. One is an accretion disk around a black hole, while the other is a system of gas like that seen in the outskirts of a quasar's home galaxy. The spectra show that their speeds relative to Earth are quite different. That either means that there are two unusual objects, several hundred million light-years apart, that just happen to line up in exactly the same direction in space, or that they are related objects that were ripped apart in the past.

The spectra and other evidence, including outbursts of X-rays, suggest the latter scenario.

Computer simulations suggest a sequence of events that began with the merger of two galaxies, each with a supermassive black hole in its core. The two black holes, which were of roughly equal mass, fell toward each other and merged. Their orbital motions and spin rates were such that the merger produced an enormous outburst of gravitational radiation. This outburst was stronger in one direction than others, so the combined black hole "recoiled" in the opposite direction. This gravitational boost kicked the black hole out of the newly merged galaxies.

Gas and stars close to the black hole were so tightly bound by the black hole's gravity that they were pulled along with the black hole. Although the merger and recoil briefly stopped material from falling into the black hole, the process quickly resumed. As gas falls into the black hole it emits X-rays and other forms of energy, creating the quasar.

The scientists who modeled these events say that black hole ejections should have been fairly common in the early universe, so many bare supermassive black holes may be roaming through space. Many other black hole mergers don't produce a strong enough outburst of gravitational waves in one direction to eject the newly-merged black hole, but instead cause it to swing back and forth through the galaxy until its motions are damped out. Searches are underway for more of these merger remnants.

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